100% Process, taskmgr.exe, PC sluggish

Hello!

For several days now, I've been dealing with my PC running extremely slowly.
Upon startup, it takes between 10 and 15 minutes to boot up.
And then, it's practically impossible for me to do anything as I have to wait 5 minutes for a simple click to take effect (not to mention launching a software).

However, here’s what I see when I manage to open the task manager:
- processes constantly using 100% (it fluctuates between 95 and 100%)
- TASKMGR.EXE between 50 and 75% (when it's not 90%)
- EXPLORER.EXE and SERVICES.EXE occupying the remaining % 15 to 20%

I know that taskmgr.exe corresponds to the task manager, but whether it is closed or not doesn't change anything, the PC still runs slow.
I tried replacing it with ProcessExplorer, but it shows the same kind of results as TASKMGR.EXE (namely PROCESSEXPLORER.exe at 50-70%, EXPLORER.EXE and SERVICES.EXE at 15-20%

However, sometimes my PC starts normally. Unfortunately, this only happens once a week, even though I try starting it 1 or 2 times a day just to see if I can use it (essentially, it starts correctly about 1 in 10 times).

When it does start normally, I take the opportunity to perform some cleanup like online anti-spyware scans (PandaActiveScan, Secuser, ...) which have removed a few infections but haven't solved the problem.
I also replaced my outdated NortonAntiVirus with AvastAntiVirus, which I have obviously launched, but it found nothing on the PC.

Note: As for deleting temporary files, emptying the recycle bin, disk cleanup, and defragmentation, I execute these regularly (1 or 2 times a week).

After several attempts to eliminate potential infections, I decided to completely format my hard drive, which I did this weekend.

On Saturday, I took care of reinstalling Windows XP, and then recovering some software downloaded from the internet, namely Avast antivirus and the ZoneAlarm firewall (which I also used before).
After rebooting, I quickly updated Windows with WindowsUpdate, then rebooted, followed by updating Windows with the installation of the SP2 pack, then rebooting again, and finally the last updates with WindowsUpdate.

It’s 2 AM and I decide to stop there for the night...
And the next day, when I start my PC, what do I see? I'm back with exactly the same problem I had before my format, with a process at 100%!!!

At this point, I admit I don't understand because the PC is completely empty, with no programs (except the antivirus, firewall, and Windows updates) and none of my documents.
So, I wonder if it’s possible that my hardware has become faulty.

I hope that with all this information, you can give me some interesting leads to resolve my issue. Thank you. Best regards.
Gwen

Configuration: Windows XP SP2
Internet Explorer 7.0
AMD Sempron 2400+
512Mb RAM

Configuration: Windows XP SP2 Internet Explorer 7.0 30GB HDD / 250MB RAM / Intel Celeron 2.4GHz / Samsung laptop